Today we look beyond Pluto/Charon toward possible ways of getting a payload to another star. Centauri Dreams readers are familiar with the pioneering work of Robert Forward in developing concepts for large-scale laser-beamed missions to Alpha Centauri and other destinations. But what if we go smaller, much smaller? Project Dragonfly, in progress at the Initiative for Interstellar Studies, proposes to explore this space, and as Andreas Hein explains below, it was recently examined in a workshop giving student teams a chance to present their ideas. A familiar figure in these pages, Andreas received his master’s degree in aerospace engineering from the Technical University of Munich and is now working on a PhD there in the area of space systems engineering, having conducted part of his research at MIT.

by Andreas M. Hein

The Project Dragonfly Design Competition, organized by the Initiative for Interstellar Studies (i4is) was concluded on the 3rd of July in the rooms of the British Interplanetary Society (BIS). To choose the rooms of the society is no coincidence. The BIS conducted the Lunar Lander study in the 1930s, which foreshadowed in an almost uncannily precise way the Apollo mission. Forty years later, in 1978, the BIS presented the first design of an interstellar probe: Daedalus. And in 2015, it was a natural choice to choose the BIS’ rooms for what might again be the stage for imagining things to come: Four international teams, almost exclusively consisting of students, are going to present their design for an interstellar probe. And this time, things get small.

Background

The field of interstellar studies can be divided into two categories. First, the field of interstellar studies is teaming with huge spacecraft, often as large and heavy as today’s largest skyscrapers. However, there is a second stream of concepts for interstellar spacecraft, beginning with Robert Forwards’ Starwisp probe and Freeman Dysons’ Astrochicken [1, 2]. These concepts stimulated thinking about the opposite: How small can we get? A small interstellar probe has a fundamental advantage. It needs less energy to accelerate to the same velocity. This is particularly relevant for interstellar missions, as we usually talk about required energies that often surpass current global energy consumption. Hence, any reduction in size might considerably increase the feasibility of an interstellar mission. Figure 2 shows an overview of some of the most relevant interstellar concepts and designs. The columns indicate the mass of a spacecraft from a particular concept or design in orders of magnitude. The rows indicate the level of detail. High-level concepts often consist of a basic feasibility analysis. System-level designs go deeper and describe key spacecraft systems in considerable detail. Subsystem-level designs add additional detail to all relevant spacecraft systems. The objective of Project Dragonfly is to address the gap in the lower left: To design an interstellar spacecraft down to subsystem level, ideally with a mass below 10 tons.

Figure 2: Some of the most relevant interstellar concepts and designs

The idea behind Project Dragonfly emerged in early 2013 when I visited Professor Gregory Matloff in New York. Greg is one of the key figures in interstellar research. That night we talked about different propulsion methods for going to the stars. We realized that nobody had yet done a design for a small interstellar laser-propelled mission. Soon after this conversation Project Dragonfly was officially announced by i4is. The name “Dragonfly” was chosen in order to pay credit to Robert Forward, who wrote the novel The Flight of the Dragonfly in the early 1980s, featuring a laser sail spacecraft. Later in 2013, i4is organized the “Philosophy of the Starship” Symposium at the BIS, where first presentations on laser-propelled interstellar probes were given by Kelvin F. Long and Martin Ciupa. Further vital preparatory work was done by Kelvin that year that fed into defining the competition requirements. With the first set of requirements defined, we finally got to the point where we were able to organize an international design competition in 2014. The purpose of the competition would be to speed up our search for a feasible mission to another star, based on technologies of the near future.

The Project Dragonfly Design Competition focused on small, laser-sail-propelled interstellar probes. Why small and laser-sail-propelled? In getting small, we are following a trend which started in the last decade. With the emergence of the CubeSat Standard first universities and then companies started to develop satellites, often not larger than a shoebox. Today, NASA and ESA are even thinking about sending small satellites to the Asteroids and Mars [3, 4]. However, the spacecraft still needs to be big enough to get enough science data back, setting a lower limit to spacecraft size, sufficient to host and supply power to the communication subsystem.

Why laser-sail-propelled? Laser sails are similar to solar sails. They both use light pressure for accelerating the spacecraft. Solar-sail-based spacecraft are today developed by various space agencies and organizations, from JAXA’s Ikaros mission to The Planetary Society’s LightSail 1. The elegance of solar sails is that they are scalable and use an abundant energy source: the Sun. Most types of solar sails could be used as a laser sail and vice versa. Hence, using a potential laser sail on a solar sail precursor would be possible in most cases. This would lower the barrier for testing a new type of sail, as operating a solar sail does not depend on a laser infrastructure.

Project Dragonfly leverages these two technology trends, as they seem to be promising to realize an interstellar mission in a scalable way.

The Project Dragonfly Design Competition

In August 2014, international university teams were invited to participate in the competition. All candidate teams had to submit solutions to a problem set first. This problem set included a range of small problems that were intended to train the teams in the key areas relevant for the competition, such as the basics of laser sail propulsion, laser systems, and in-space communication systems. The objective of this initial problem set was two-fold. First, it was intended as an entry barrier for all teams that do not have a serious intention or the capabilities to participate in the competition. Eliminating teams that would not make the cut later on also had the purpose of avoiding overburdening the reviewers. The reviewers we invited are very busy individuals. We wanted to use their time as effective as possible, giving them the opportunity to focus on the best teams. Second, the successful teams would be able to develop or strengthen their capabilities to solve the main competition task of designing a laser-sail-propelled interstellar probe. They would also get familiar with the existing literature on the topic and get a “feeling” for the subject.

The teams that were able to pass this hurdle were then confronted with the mission requirements. These requirements used the requirements developed during Project Icarus as a starting point. Project Icarus is an ongoing collaborative interstellar study between the BIS and Icarus Interstellar, in which I am participating [5]. However, the requirements were adapted and extended to the laser sail case. The requirements are depicted in Figure 3 in a graphical fashion.

1. To design an unmanned interstellar mission that is capable of delivering useful scientific data about the Alpha Centauri System, associated planetary bodies, solar environment and the interstellar medium.

2. The spacecraft will use current or near-future technology.

3. The Alpha Centauri system shall be reached within a century of its launch.

4. The spacecraft propulsion for acceleration must be mainly light sail-based.

5. The mission shall maximize encounter time at the destination.

6. The laser beam power shall not exceed 100 gigawatts

7. The laser infrastructure shall be based on existing concepts for solar power satellites

These requirements were deliberately fine-tuned in order to be challenging. The 100 GW beam power requirement constrains the design space considerably. The particular value was selected as it constrains the mass of the spacecraft to tens of tons. Furthermore, it is a beam power that is very challenging to generate with an in-space infrastructure within the 21st century but not completely out of reach. The 100 year time constraint sets a theoretical minimum average trip velocity of 4.3% of speed of light in order to reach the Alpha Centauri star system. With the power constraint only the spacecraft mass, its sail system parameters, and the duration of acceleration / deceleration are left as key variables. A long acceleration duration allows for reaching a high velocity. However, a long acceleration duration means that the laser beam has to be steered over long distances. This in turn makes pointing and focusing the beam challenging.

The science data requirement is also challenging to fulfill. If the teams decide to reduce the spacecraft mass, they need to shrink their communication system as well. However, communication over interstellar distances requires large amounts of power, if useful science data is to be collected and sent back.

The teams needed to navigate in this design space, making careful trade-offs between different parameters. The competition included two intermediate stage gates and a final review of the reports. Each stage gate required a different set of deliverables that are commonly required for concept studies in the space domain, such as an initial feasibility analysis, a technology readiness assessment, and detailed calculations for key aspects of the mission. The stage gate process allowed us to check and adjust our expectations for the next stage of the competition and provide targeted support if teams were struggling in a particular area. Furthermore, each of the deliverables covered a vital aspect that is commonly needed for a concept study. The staged approach enabled the teams to work on a limited set of deliverables at each stage, reducing the difficulty of the overall task.

The competition wouldn’t have been possible without our reviewers and advisors. Fortunately, we were able to recruit experts with considerable experience in solar and laser sailing studies, such as Les Johnson, who is working as the Deputy Director of the Advanced Concepts Team at NASA and Bernd Dachwald, a German professor.

Four international teams, out of initially six contestants were able to take all hurdles and submit a final design report:

Technical University of Munich
University of Cairo
University of California Santa Barbara
CranSEDS, consisting of students of Cranfield University, UK, the Skolkovo Institute of Science and Technology in Russia, and the Université Paul Sabatier in France.

These reports were this time graded by the reviewers. Furthermore, the teams were invited to present their designs at the final workshop in London, on the 3rd of July 2015.

The workshop

The main purpose of the workshop was to mimic a typical design review in the space sector. The teams would give a presentation, covering all vital aspects of their design and would then answer questions from the audience and review panel. The review panel consisted mostly of aerospace engineers. Notable members were the Executive Director of i4is, Kelvin F. Long, and Chris Welch, who is working as a professor at the International Space University.

The first team to present was the team from the Technical University of Munich. Their spacecraft would be accelerated up to a distance of 2.2 light years and a velocity of 9% of the speed of light. The laser infrastructure would be placed on the Moon. Their laser sail would consist of a graphene sandwich material. Deceleration is enabled by a staged magnetic / electric sail. The team chose this staged approach, as the magnetic sail is very efficient at high velocities but gets increasingly less efficient at lower velocities. The electric sail is capable of decelerating at lower velocities. The spacecraft reaches the Alpha Centauri system after 100 years. Communication is enabled by a laser communication system. Power is supplied, either by solar cells that generate energy from the laser beam or the electric sail, which generates electric power when flying through the interstellar medium.

Figure 4: The spacecraft of the Technical University of Munich. Sail is not to scale.

The overall spacecraft is relatively heavy, compared to the other designs. It’s mass is 14t. Part of the reason is that the team aimed at maximizing the payload mass. A higher payload mass leads to a higher scientific yield but also leads to a heavier communication system, due to the higher data rate. Another effect is that a heavier spacecraft needs a longer duration to accelerate, imposing pointing requirements on the laser optics that are difficult to meet. Another difficulty with the overall architecture of the mission is that the laser system is located on the lunar surface. Although in principle feasible, installing such a system is very costly, unless a large amount of in-situ materials are used.

Figure 5: UCSB team wafer spacecraft design

The second presentation was given by the UCSB team. This team’s design combined the highest number of innovative technologies. It distinguished itself in numerous ways from the other teams. First, the concept of the spacecraft was a “wafer-based” design. This means that the spacecraft is basically imprinted onto a chip with all spacecraft subsystem integrated into it. The sail would consist of a dielectric material with an extremely high reflectivity, in order to withstand the enormous power density of several gigawatts per square meter. Note that sunlight in Earth orbit has a power density of about 1.4kW per square meter. Hence, the power density of the laser is roughly a million times higher than what today’s spacecraft are usually facing.

A highly reflecting surface avoids that part of the energy that is absorbed by the spacecraft, immediately melting it. The spacecraft is also accelerated rapidly, within a distance of three astronomical units, up to a velocity of 25%c. The team was able to consider such high velocities, as the spacecraft does not contain any deceleration system. Using deceleration systems such as a magnetic sail or an electric sail would lead to significant deceleration durations that may nullify any decrease in trip duration as a result of the high cruise velocity. However, the lack of a deceleration system does not comply with the mission requirements. The laser is a phased-array fiber-fed laser.

Figure 6: Samar Eldiary presenting the Cairo Team’s spacecraft

The third presentation was given by the Cairo University Team. The team’s mission design is based on an initial acceleration via laser beam, based on the DE-STAR system developed by the UCSB team. The spacecraft is decelerated via magnetic sail, and then separates into two sub-probes. One probe will collect data from Proxima Centauri, the other data from the Alpha Centauri A and B system. The laser sail is made out of aluminum. A laser communication system is used for sending back data to the Solar System. Power is provided by three Radioisotope Thermoelectric Generators (RTGs). The team presented an innovative approach for attitude control during the acceleration phase by changing the shape of the sail.

Figure 7: CranSEDS spacecraft design. The left image depicts the spacecraft bus with payload. The three cylindrical objects are the RTGs.

The last presentation was given by the CranSEDS Team. The interesting thing about their mission architecture is the use of a staged approach. A total of three spacecraft are launched within 33 year intervals. The rationale behind these intervals is to use each subsequent spacecraft as a communication relay station as well as exploiting technological advances that have occurred in the meantime.

First, the spacecraft is accelerated up to a velocity of 5%c at a distance of about 5,800 astronomical units. This phase takes about 3.7 years. The laser sail, used during acceleration, is then jettisoned. The subsequent cruise phase takes up to 77.5 years. Deceleration then starts via magnetic sail and the spacecraft enters the Alpha Centauri system after about 98 years of flight. 33 years after launch, a second spacecraft is launched with a similar configuration, but mission phases shifted by 33 years. The third spacecraft is launched after a similar interval of 33 years.

The team’s spacecraft is propelled by a Silicon Carbide sail. Power is provided by three RTGs. Data is sent back via laser communication. The overall mass of one of the spacecraft is about 4.5 tons. Each spacecraft hosts a scientific payload of 93kg, consisting of various instruments such as spectrometers, a magnetometer, and a cosmic dust analyzer.

The team presented detailed trade-off analyses for each of the critical aspects of the mission such as how many spacecraft to send and each of the spacecraft subsystems. The reviewers, however, remarked that the mission architecture, consisting of three separate spacecraft might induce programmatic risks, as the mission would need to be sustained over a period longer than a century. Furthermore, the so-called waiting paradox might come into play: A spacecraft launched later could overtake an earlier one due to a more sophisticated technology or laser infrastructure.

After the teams’ presentations, the review panel retreated for ranking the teams. As mentioned earlier, the team reports already contributed to the overall grading with two-third of the points. One-third would consist of the presentation and the teams’ performance during the Q&A session. After a few discussions, the review panel reached a conclusion and got back to the teams, waiting eagerly to hear the results.

The first prize, which went to the team of the Technical University of Munich, went along with one of the Alpha Centauri Prizes, which i4is awards to contributions advancing the field of interstellar travel.

After the ranking was announced, the next steps for Project Dragonfly were presented. First, the teams are requested to submit a summary of their report to a peer-reviewed international journal. The purpose is to receive another independent validation of the designs. Furthermore, the teams would gain experience in writing scientific publications. Another step is a technology roadmap, based on the technologies that were selected by the teams. Some of the technologies were common, such as laser communication and a magnetic sail for deceleration. However, the teams diverged in other technologies such as the laser sail material and power supply. With the teams, we will select key technologies and think about what steps are needed for developing them, along with prototypes and demonstration missions.

Later in the afternoon, the workshop participants gathered at the local bar and restaurant, the Riverside: A traditional gathering place after BIS events. Here, new friendships were forged between the participants and the future of Project Dragonfly was hotly debated.

Conclusions

The main conclusion is that a small, laser-sail-propelled interstellar mission is in principle feasible by using a laser infrastructure providing a 100GW laser beam. The Alpha Centauri system could be reached within 100 years. The spacecraft mass would be somewhere between 15 and a few tons. With the use of innovative technologies, even masses below one ton could be achieved.

The following conclusions can be drawn from the presented spacecraft designs:

Laser communication seems to be a promising approach for achieving communication over interstellar distances.

Magnetic sails seem to be the currently most promising way to achieve deceleration from velocities of a few percent of the speed of light.

The trade-offs for the best laser sail material are non-trivial and there seem to be several promising materials.

Most teams have used RTGs as power supply.

More research needs to be done on the laser infrastructure. In particular, where to place it and how to leverage on potential future solar power satellite infrastructures.

However, there are several other feasibility issues that need to be addressed, such as beam pointing requirements over distances of several to thousands of astronomical units. Manufacturing and deployment of kilometer-sized solar sails is also an issue. Furthermore, spacecraft autonomy during the mission is a huge challenge as well. Deployment of magnetic sails with several kilometers in radius remains another feasibility issue.

Despite these challenges, let us not forget where we came from: Missions using the whole energy consumption of humankind. We were able to decrease that by two orders of magnitude or more. Yes, building such an infrastructure is an immense challenge but it is less a challenge than for example mining Jupiter for Helium 3 for two decades, as proposed for Project Daedalus, or harvesting large quantities of antimatter.

Maybe, and just maybe, some of the ideas presented during the workshop might one day open up the pathway to the stars. Until then, a lot of work remains to be done.

This is a human thing rather than a technical one (I’ll get to those below), but I was so heartened to see a young Muslim woman giving a presentation on an interstellar probe design, and working happily alongside a western woman who was *not* covering her hair! That’s the kind of future I want to see happen, where people–thinking of themselves in relation to the vastness out there–stop caring so much about their differences and just work and live together without strife. And one day, hopefully, similar pictures will include human beings and their alien colleagues (whether they’re together in person or via a pictorial transmission from afar, no matter how “light time-lagged”)…

The probe designs are very interesting, and are useful not only as stepping stones to what will one day ride a light beam to the Alpha Centauri system, but also for an equally important psychological reason. Once an image becomes fixed in an organization’s collective minds, no alternatives–no matter how good–can gain a foothold. Sail propulsion, while popular here among this group of enthusiasts, is still considered “strange” and “exotic” in the aerospace engineering community at large, many of whose members remain skeptical about its practicality. Here’s what I’m getting at:

An example: Once NASA decided in the 1970s that its new Space Shuttle would be a winged vehicle, nothing without wings was even considered. Chrysler (which built the Redstone and Jupiter missiles and the Saturn I and Saturn IB first stages) proposed a reusable Single-Stage-To-Orbit, plug-nozzle vehicle called SERV, which studies then (and later) showed would work. But because it had no wings, NASA passed it over with barely an acknowledgement, even though this conservative design would have been more versatile than a winged craft. Now:

Getting the sail concept into peoples’ minds as a viable contender (*not* to the exclusion of other concepts, but as another accepted and respected way to get to the stars) will go a long way toward facilitating its realization. The combination of laser, magnetic, and electrostatic sails is even more positive, since the latter two concepts are even more “fringe-ey” than the laser sail, outside the interstellar enthusiast community. In other words, getting viable and inspiring archetypes into the minds of the people today–and their successors of tomorrow, who will one day build them–will ensure that these sail-based interstellar probe concepts get the research and development “attention” that they will need in order to be realized as working hardware.

Fascinating alternative designs! It’s particularly interesting how different the four concepts are from each other, despite the design constraints.

I feel that kinetic impact powered pulse propulsion offers the best way to decelerate at Alpha Centauri. It lets you continue to utilize the laser acceleration system after the probe has left effective range or has gotten roughly halfway there. I’m wary of large scale laser sails, but a swarm of small laser sail drones could effectively accept momentum from a laser beam on the way out, and then power the braking run by colliding with puffs of inert on board propellant to the main spacecraft.

For this mission, the main spacecraft could be just a mag-loop with a couple payload modules on opposite points along the loop.

I think it is really great that these designs seem feasible with technology that isn’t unrealistic. I hope that these designs get published so that their concept details can be disseminated.

While the goal is alpha Centauri in a century, I also think that the performance of these sail probes should be considered for exploration of the outer solar system. For example the Cranfield team has their probe reaching the Oort cloud in under 4 years. The UCSB team has their probe traveling at 0.25c in just 3AU! This would dramatically shorten exploration times and with multiple flybys rather than orbiters, we might get a lot of science done quickly and cheaply with these sorts of approaches. One the laser platform is built, it can be used for many missions, amortizing its costs, and many similar probes should reduce unit production costs as well as tolerate probe failures due to the high redundancy of probe numbers. Solar system exploration would also be the proving ground to develop technologies and probes before interstellar flights are started.

I think this could be the start of an exciting approach that could transform our approach to pathfinding planetary exploration.

Have an AXIS simulation back home, preferably in the San Diego area, and call it Jill. The question then is which becomes self-aware first, Jill or AXIS? If AXIS, it might become quite lonely by itself in the Alpha Centauri system. So, you’ll want to send two of them.

Exciting and we should be able to do this one this century . It seems to me the laser is the big issue. Anyone thinking about a moon base.? I Think the Russians brought it up. This would also be something we could focus a manned program around.?

Alex, that is another “technological archetype image” (first utilizing these various laser [and perhaps maser, too], magnetic, and electrostatic sail propulsion/braking systems for quick image & data return missions to the outer solar system) that we need to put into the minds of the interested public, the aerospace engineering community, and space agencies. Also:

Such missions, utilizing a number of small, successive probes that fly by their targets over a rather extended period, could be called “Continuous Flyby” missions, which–as you wrote–would be about as good as orbiters. (They would even offer an advantage over orbiters–each flyby probe would return *both* far- and close-range imaging and fields & particles data, while an orbiter only collects far-range images and data once, as it approaches its target world.) In addition:

Once such outer solar system applications of these sail probe technologies are being developed as hardware–or even studied in detail, in preparation for this–the interstellar applications of reasonable extensions of these technologies will naturally occur to many people, from visionaries to the most hard-boiled, practical engineers (“Hey…we *can* actually send probes to the stars using these!”). Plus:

As far back as 1968, Arthur C. Clarke suggested–in his non-fiction book “The Promise of Space”–that such externally-powered vehicles (although he didn’t specifically mention sails) might overcome the mass ratio (and thus maximum velocity) problems of starprobes and starships that carry their own energy sources (even reaching 0.1 c was considered wildly optimistic for such “orthodox” spacecraft, as he wrote above the following quotation):

“It is at least conceivable that the interstellar ramjet may work, *or that it is possible to supply power from an external device, such as a planet-based laser*, or that the universe contains still unknown sources of energy which spacecraft may be able to tap.”

Back then the external power option was considered pure speculation, physically possible but likely impractical. The default “probable” starflight concepts then all involved staged fission or fusion rockets of some kind, which isn’t surprising, since they were all extensions–although long ones–of chemical rocket technology, throwing mass in one direction in order to obtain motion in the opposite direction. The idea of having an “engine” (a laser or maser) that is separate from the vehicle had no “experience base” from which to be extrapolated, as the nuclear rocket-powered starship did. Now is the time to begin creating that “experience base” for the beam-pushed interstellar sail probe.

‘Deployment of magnetic sails with several kilometers in radius remains another feasibility issue. ‘

This is surprisingly easy, the magnetic field been switched on can have the effect of opening the coil naturally.

It might be better to launch a few probes toward the target star to form a wide base transmitting and optical system, also using the target stars gravitational lens effect on the other side of the star could reduce the transmitting power needed. Also a few probes in the opposite direction of the target star somewhat later towards the Suns focal point would reduce the communication transmitting powers considerably as well.

Now the probes would be the least expensive part of the system so many could be built. I was thinking along the line of having each probe embedded in a part of the sail that was detachable just before the acceleration phase ends to give the wide base needed as they moved apart.

I am sure someone has thought about it but why don’t they add just enough propellent to flip around the sail and engine/exhaust once it gets closer to another star? Use the solar energy of that star and decelerate with flipped exhaust. You can put in automated instructions to trigger that flip.

for a while its seemed to me that space craft that could go a modest fraction of the speed of light (lets say .01-.1c) have been within the reach of modern technology. Fission fragment, some Orion proposals, laser sails. I hope in the next few decades something like this is built. also .25c? damn thats amazing it would be a real game changer to make something that moves 100th as fast!

LETS GET STARTED as Paul said. It looks to me the biggest issues are the laser and the communication. I wondered how big anissue the laser is? I read we have reached that power but I amno epert and wondered how bigit would be and how it could be powered there…solar a nuclear plant both?………Our Orion is a heavy launch that can get to the Moon …so a moonbase and laser does not seem far fetced even from funding.A lot of the congresscritters really wanted a moonbase. This wouldbe the reason we needed

Deceleration at the target star system, while a good thing, isn’t strictly necessary, especially if a large number or cheap flyby laser sail probes (the “sail-chip” types appear to be particularly well-suited to this) could be sent as a “string.” While no -one- probe would get more than a few very brief observations of the system, a “continuous flyby” string of such probes–which could be targeted to pass through the system on different paths (rather like the Pioneer Venus atmospheric probes, which “bracketed” the planet’s Earth-facing hemisphere)–could provide photographic and data coverage of the whole system. Also:

A Moon-based laser launching system (Larry Niven’s short story “The Fourth Profession” touches upon this) could be justified (and perhaps even pay for itself) via other things that it could do. These would include (but need not be limited to):

[1] Launching cheap sail probes to the outer planets (or really, to any destination in the solar system);

[2] Launching crewed sailcraft to cislunar and solar system destinations;

[3] Providing beamed power to mining operations in the outer asteroid belt where the sunlight is weak (and to such mining operations on Near-Earth Asteroids, when their rotations carry the mine sites around to their night sides);

[4] Providing local climate modification/weather control on Earth (I’m not advocating this, just listing it as a possibility–we’ll need to learn much more before trying such things, -if- we do them);

[5] De-orbiting space junk around the Earth via laser-heated ablation/retro-thrusting on the unwanted objects (this has been proposed, using Earth-based lasers);

[6] Altering the orbits of potentially-hazardous objects (asteroids and comets) which could strike the Earth, via the same laser-heated ablation/thrusting method that could be used to de-orbit space junk (the basic principle is the same–it would require more power and/or longer “beaming” periods to nudge Earth-crossing asteroids and comets into ‘Earth-missing’ paths), and;

Two laser stations for doing these things–as well as for launching interplanetary and interstellar probes–could be established, at the Moon’s poles. The lasers’ heat radiators could be situated in permanently-shadowed craters (perhaps suspended above their floors, in the perpetual darkness), where the temperatures are the coldest ever measured in the solar system. Together, the two antipodal laser (or laser array) stations could “see” the entire celestial sphere, so their beams could be directed to any point in space, on Earth, or on (or near) any planet, satellite, asteroid, or comet in our solar system. The Moon’s slow rotation would make aiming (and maintaining the aim of) the lasers easy. Also:

While such laser stations could, of course, be used as weapons against spacecraft and targets on the Earth’s surface (or flying in its atmosphere), nations on Earth could easily retaliate in kind with lasers, or hurl nuclear warheads at the lunar laser stations (they could be shielded well enough [perhaps by being provided with thick reflective surfaces] to withstand the laser “glare” for long enough to hit their targets). Even that might not be necessary, as the nation on Earth that started the laser “war” from the Moon could be attacked directly by its rivals on Earth. I think that even given how the world is today, engaging in such a science fiction scenario–in real life–*wouldn’t* appeal to national leaders. Let’s ride those beams to the stars instead!

“A lot of the congresscritters really wanted a moonbase. This wouldbe the reason we needed.”

Really? I recall with pain when Newt Gingrich suggested a manned lunar base by 2020 during the 2012 US Presidential campaign the response was mocked and derided on both sides of the political fence at virtually all levels. Mitt Romney even infamously said in response he would fire anyone working for him who even dared to suggest such an idea.

So who are these congressmen and why aren’t they being more vocal about funding a Moon base? Can they really be counted on to support other areas of the space program?

What would be the power levels of the RTG’s after 98 years of flight?
you might have to capture data and then transmit data separately AKA new horizons
Those 98 year old RTG’s will have to power a 4.5 light year laser transmitter and the spacecraft avionics and science payloads

@Qi the reason is that the mass ratio is still enormous. Assume a mid flight velocity at 5% c. Assume a really high Isp of 100,000 – some sort of advanced ion engine. The propellant needed would be millions of times larger than the payload, all of which would have to be accelerated by the sail.

A fission fragment rocket might get you an Isp in the 10^6s range, which might be more feasible, but then why use the solar sail?

I am also interested in what is needed to send back the data from 4.5 ly away, and what the data rate would be. I presume that is partially dependent on the receiving stations on Earth. What are the assumptions? Would a flyby at fractional c work for making observations and beaming back the data, or must we decelerate to make this work?

Qi, what you described is called a “Sun-diver” (‘star-diver’ in this case) or “dive and fry” sail mission. Dr. Gregory Matloff, James Essig, and others have studied such missions. While the engineering requirements are enormous (and beyond our abilities at present), it appears to be possible to use a sail that would deploy *very* close to the Sun behind an occulter, be accelerated up to as much as 25% of c (the speed of light) within hours, and then decelerate to a stop in the target star system by diving in toward the star. (It might hover above the star’s surface as a statite before tilting its thrust vector to enter an orbit, or it might target a point very close to the star in order to directly enter an initial orbit from its fast interstellar cruise–either way, “staring Alpha Centauri A in the eye” upon arrival would be an awe-inspring sight!) Also:

While it’s possible that starships might travel to the nearer stars in this way (the occupants would have to be immersed in liquid, with breathing apparatus, during the crushing departure acceleration and arrival deceleration phases), pushing laser probes up to such velocities (over a longer period) using lasers is a gentler way to “get up to speed” (magnetic and electric sail braking are, likewise, gentler ways to slow down on arrival) that current (or soon-to-be-in-hand) technologies will be able to do.

“Alex, that is another “technological archetype image” (first utilizing these various laser [and perhaps maser, too], magnetic, and electrostatic sail propulsion/braking systems for quick image & data return missions to the outer solar system) that we need to put into the minds of the interested public, the aerospace engineering community, and space agencies. Also:

Such missions, utilizing a number of small, successive probes that fly by their targets over a rather extended period, could be called “Continuous Flyby” missions…”

I am very surprised to learn that sailing concepts haven’t gained greater acceptance amongst engineers. While there’s not much experiance base there, it circumvents so many fundamental physical poblems of energy and mass that it’s hard to imagine a near/mid term interstellar probe (that wouldn’t be unfeasibly expensive to build) that doesn’t ‘leave the engine behind’ in some way.

A ‘train’ of small fliyby probes might also mitigate some of the problems of returning communications to Earth, as each probe in the train could act as a relay station.

“Back then the external power option was considered pure speculation, physically possible but likely impractical. The default “probable” starflight concepts then all involved staged fission or fusion rockets of some kind…”

If I might speculate along those lines (the physically possible but likely implausible), I have often woindered about the idea of a very different kind of ‘probe’: Would it ever be concievable to control and focus a pulse of radiation well enbough to use it to actively (as opposed to modern telescopes that are passive) scan objects across interstellar distances, in the same way that facilities such as Arecibo today scan objects across interplanetary distances? This might seem an absurd suggestion given the distances involved, but then I imagine that radar scanning the poles of mercury from Earth would be absurd to a 1940’s radar engineer. Such an radiationn based ‘probe’ would never have the utility of a physical probe, but might be a useful (and less expensive) way to gain data about other star systems and compliment the facilities of a physical probe.

This just in! Closest CONFIRMED transiting exoplanet (21.5 ly) orbiting Gliese 892 (or HR8832, take your pick). Unless Alpha Centauri Bc is confirmed LATER ON, this MAY be the closest transit we will EVER be able to observe! Could a PROJECT DRAGONFLY small probe make it that far?

@jason Wentworth it appears to be possible to use a sail that would deploy *very* close to the Sun behind an occulter, be accelerated up to as much as 25% of c (the speed of light) within hours

Are you sure of those figures? If a sundiver could reach 25%c, why use a laser at all? The sail should be able to both accelerate and decelerate around stars. Sundivers will certainly increase velocity of the sail, and this can be further increased with beaming, but I would like to see a reference concerning sundiving sails reaching 25% c.

Excellent work for University students and a well-written report. The only thing missing that I detected was the fact that interstellar space is not empty and the probes could expect several solid particle impacts per second.

John, many aerospace engineers still consider sails (which are admittedly flimsy things) to be very tricky to deploy and maneuver without getting them all tangled up (IKAROS’ controllers used a two-step deployment sequence, with a few days of observation and analysis between them, because of these concerns). Regarding scanning other planetary systems from Earth:

I’m reading John MacVey’s book “Space Weapons, Space War” at the moment (I heartily recommend *all* of his books on SETI, interstellar travel, space colonization, and even time travel), and he wrote that despite the immense distances involved, conducting radar astronomy at interstellar distances is not impossible, although it’s beyond *current* human technological capability; he suggested that using other portions of the electromagnetic spectrum might work (and I conjecture that using gravity waves–if/when we are able to detect and generate them–might permit such ultra-long-distance scanning). Also:

[1] the *extreme* acceleration (and deceleration, if used upon arrival–it/they would take place over just a few hours; the required engineering isn’t impossible, but it’s beyond current capabilities);

[2] The fantastic heat flux that the sail and payload would have to deal with–refractory materials would be mandatory and their selection would be non-trivial, and;

[3] Avoiding billowing and ripping of the sail under such a powerful photon pressure, so close to the Sun and the destination star. This problem isn’t insurmountable either, but its solution wouldn’t be anywhere near easy. Nevertheless:

If some dire need made it necessary to send a starprobe or a starship to one or more of our stellar neighbors (to find a new home and escape there if our Sun showed signs of going nova, perhaps?), a crash program could probably make such totally “naturally-propelled” high-velocity star-traveling spacecraft feasible within this century.

Like @AlexTolley the idea of a swarm intrigues me. Imagine a New Horizons type flyby intra-solar system mission releasing a swarm of probes that were little more than metaphorical ‘GoPro’ cameras. If some of these probes led the main mission while some followed those before along the same path and if they were spread in time to match the rotational period of the target planet then you could stitch together a ‘bullet time’ full virtual orbit of the target planet.

Each probe would communicate with the main mission which has the large transmitter to send the resulting images home.

I also liked the staging idea used by the CranSEDS team for interstellar missions – but instead of separate launches I’d consider it better to fragment the originally launched probe – reducing mass as the laser beam’s focus reduces but leaving a trail of booster transmitters to improve communications. These boosters would also do good science and eventually reach the target but they could also act as trail blasers – providing staging transmitters for later probes. There might be a cost to consider however… is it worth such a transmission improvement when paid for the cost in initial mass that may never reach the target?

I really love this idea of small probes pushed to high velocities with super powerful lasers stationed here in the Sol system. We could build many lasers directly converting the sun’s energy into tightly focused light and send probes to all our neighbor stars. The best is that the lasers could have dual purposes. Primary purpose: to push probes outwards, Secondary purpose: Sol system defense against possible hostile aliens. Hopefully we will never have to use the secondary purpose but I would think, given our vulnerability, that it is better than nothing.

‘Primary purpose: to push probes outwards, Secondary purpose: Sol system defense against possible hostile aliens. Hopefully we will never have to use the secondary purpose but I would think, given our vulnerability, that it is better than nothing.’

Lasers have a big advantage in speed and power but as a defence against Alien attack they would be very poor as all they have to do is have a good reflector material and worse they could reflect it back at us! The light might blind the electronics though.

I agree, Jim Savage. “Sun-pumped” lasers (as opposed to internally-powered ones) would neatly side-step the power supply problem by using our natural, nearby fusion reactor. This type of laser technology should be pursued vigorously, because of its immense and varied potential. Also:

The Brobdignagian laser array (suspended near Mercury by a statite solar sail) that Dr. Robert Forward designed to propel the 1,000 km diameter (he always “thought big” :-) ) laser sail starship in his novel “The Flight of the Dragonfly” could–in a greatly scaled-down (and possible for us to build today or soon) form–be used to propel interstellar sail probes and to gradually alter the orbits of potentially hazardous asteroids, as well as to beam photonic power to “laser-tuned” photovoltaic arrays on the Earth at night (particularly during the months-long polar nights). Any nation foolhardy enough to try to “fry” targets on Earth using such lasers would find itself attacked directly by its adversary or adversaries, so that’s one science fiction scenario that I don’t lose any sleep worrying about.

I am thinking a fission fragment sail combination could be better. If we had a nuclear reactor which created energy for the laser/microwave beam and we stored the radioactive waste in a circular particle accelerator we could take advantage of the time dilation effect to greatly increase the half-life of the radioactive waste. We could then later spray this waste or selected parts of it onto the unfurled sail and still use the laser/microwave beam to propel it. Fission fragment sails have great ISP’s.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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